A string having the format
file://path/to/file.pem - the named file must
contain a PEM encoded certificate/private key (it may contain both)

A string containing the content of a certificate/key, PEM encoded

For private keys, you may also use the syntax
array($key, $passphrase) where $key represents a
key specified using the file:// or textual content notation above, and
$passphrase represents a string containing the passphrase for that
private key